Quantum chip rides on superconducting bus

For the first time the components that underlie quantum computing's great potential - qubits - have been linked on chips like those in conventional computers. Two US research teams used superconducting circuits to make two of the quantum components linked by a quantum information cable or bus.

The bits that work together on calculations in a normal computer can exist in two states - either 0 or 1. But qubits can inhabit both at once, allowing them to process many calculations simultaneously when they get together.

Qubits can be made in different ways, but so far most schemes don't lend themselves to mass manufacture. Putting them onto chips like those in conventional computers could unlock the economies of scale that have driven the rise of electronic computing.

Separate studies reported by two groups in this week's Nature journal suggest that could be possible. One is at the US National Institute of Standards (NIST) in Boulder, Colorado, and the other at Yale University in New Haven, Connecticut, both in the US. The teams say they have connected qubits on a chip for the first time.

Quantum bus

Each group made qubits from 'artificial atoms' - tiny clumps of around 1 billion aluminum atoms with discrete electrical energy levels just like a single atom. They linked them with a "cavity" that can shuttle information between them. The whole apparatus was cooled to a few thousandths of a degree above absolute zero to make the circuits superconducting.

The cavity is lined with an electrical insulator that traps a single microwave photon that bounces around inside in a standing wave. Using pulses of microwaves, information can be encoded as electromagnetic energy in the artificial atoms, and then transferred to and from the cavity.

In effect, says Johannes Majer, a member of the Yale team, the researchers have created "a quantum bus". A bus is used in conventional computers as a conduit for information among the various components - but its quantum chip equivalent has never been made before.

Going a step further, the NIST group showed the cavity can be more than just a bus - they used it as short-term memory. After encoding information in one qubit, they transferred it into the cavity for 10 nanoseconds before transferring it to the other qubit.

Yale's chip used qubits around 1-micron square built on silicon, while NIST used larger 10-square-micron qubits on top of sapphire. In both prototypes, the bus between the qubits was between five and seven millimeters long.

Key elements

Taken together, the groups have demonstrated the key elements needed to perform the basic logic operations that underlie all computing. Raymond Simmonds who led the NIST team says future quantum chips will probably use some version of it. He calls it "the nerve center" of the quantum chip.Future research by both groups will focus on connecting more qubits with on-chip cavities. At present adding more is difficult because of problems with the stability of information in them. "We are preparing an experiment with 6 qubits, but there is room for more qubits [on a chip]", says Majer.

Quantum computing experts Antti Niskanen of the VTT Technical Research Center of Finland and Yasunobu Nakamura of NEC Nano Electronic Laboratories in Japan agree. "There is, in principle, room for hundreds of qubits on the same chip," they write in a commentary accompanying the two papers. "Because any pair of qubits can be coupled, implementing quantum algorithms and error-correction codes in a quantum computer will be significantly easier."

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